The plasma level of NOx, i.e., the sum of NO2 ؊ and NO3 ؊ , is frequently used to assess NO bioavailability in vivo. However, little is known about the kinetics of NO conversion to these metabolites under physiological conditions. Moreover, plasma nitrite recently has been proposed to represent a delivery source for intravascular NO. We therefore sought to investigate in humans whether changes in NO x concentration are a reliable marker for endothelial NO production and whether physiological concentrations of nitrite are vasoactive. NO 2 ؊ and NO3 ؊ concentrations were measured in blood sampled from the antecubital vein and brachial artery of 24 healthy volunteers. No significant arterial-venous gradient was observed for either NO 2 ؊ or NO3 ؊ . Endothelial NO synthase (eNOS) stimulation with acetylcholine (1-10 g͞min) dose-dependently augmented venous NO 2 ؊ levels by maximally 71%. This effect was paralleled by an almost 4-fold increase in forearm blood flow (FBF), whereas an equieffective dose of papaverine produced no change in venous NO 2 ؊ . Intraarterial infusion of NO2 ؊ had no effect on FBF. NOS inhibition (N G -monomethyl-L-arginine; 4 -12 mol͞min) dosedependently reduced basal NO 2؊ and FBF and blunted acetylcholine-induced vasodilation and NO release by more than 80% and 90%, respectively. In contrast, venous NO3 ؊ and total NOx remained unchanged as did systemic arterial NO 2 ؊ and NO3 ؊ levels during all these interventions. FBF and NO release showed a positive association (r ‫؍‬ 0.85; P < 0.001). These results contradict the current paradigm that plasma NO 3 ؊ and͞or total NOx are generally useful markers of endogenous NO production and demonstrate that only NO2 ؊ reflects acute changes in regional eNOS activity. Our results further demonstrate that physiological levels of nitrite are vasodilator-inactive.endothelium ͉ blood flow ͉ red blood cells ͉ endothelial dysfunction

Abstract-Higher doses of inhaled NO exert effects beyond the pulmonary circulation. How such extrapulmonary effects can be reconciled with the presumed short half-life of NO in the blood is unclear. Whereas erythrocytes have been suggested to participate in NO transport, the exact role of plasma in NO delivery in humans is not clear. Therefore, we investigated potential routes of NO decomposition and transport in human plasma. NO consumption in plasma was accompanied by a concentration-dependent increase in nitrite and S-nitrosothiols (RSNOs), with no apparent saturation limit up to 200 mol/L. The supposedly rapid conversion of NO to biologically inactive metabolites in human blood formed the rationale for inhalation NO therapy, because the short half-life of NO should confine its effect to the pulmonary circulation. 4 However, recent evidence suggests that higher doses of inhaled NO may exert side effects beyond the pulmonary circulation. 5,6 Red blood cells (RBCs) are believed to be a major sink for NO by virtue of the rapid co-oxidation reaction of NO with oxyhemoglobin to form methemoglobin (metHb) and nitrate. Alternatively, NO may react with hemoglobin (Hb) to form either nitrosylhemoglobin (NOHb) or S-nitrosohemoglobin (SNOHb). 6,7 In addition to its reaction with RBCs, NO has to interact at some stage with plasma constituents, especially in view of the existence of an RBC-free zone close to the vessel wall. 8 A better knowledge of the fate of NO in plasma is an important prerequisite for a proper understanding of its physiology in blood and in the human circulation in general.Recently, we provided evidence that intra-arterially applied NO can be transported in a bioactive form over significant distances along the forearm circulation. 9 To date, no data have been reported on the systemic dilator effects of intravenously applied NO in the human peripheral vasculature, presumably because of its supposedly rapid clearance from blood. Our data challenge this current dogma. In the present study, we demonstrate that the intravenous infusion of NO solution results in the transport and delivery of NO as S-nitrosothiols (RSNOs), which are accompanied by systemic hemodynamic effects and vasodilation in conduit and resistance vessels.

Although hitherto considered as a strictly locally acting vasodilator, results from recent clinical studies with inhaled nitric oxide (NO) indicate that NO can exert effects beyond the pulmonary circulation. We therefore sought to investigate potential remote vascular effects of intra-arterially applied aqueous NO solution and to identify the mechanisms involved. On bolus application of NO into the brachial artery of 32 healthy volunteers, both diameter of the downstream radial artery and forearm blood flow increased in a dose-dependent manner. Maximum dilator responses were comparable to those after stimulation of endogenous NO formation with acetylcholine and bradykinin. Response kinetics and pattern of NO decomposition suggested that despite the presence of hemoglobin-containing erythrocytes, a significant portion of NO was transported in its unbound form. Infusion of NO (36 µmol/min) into the brachial artery increased levels of plasma nitroso species, nitrite, and nitrate in the draining antecubital vein (by < 2-fold, 30-fold, and 4-fold, respectively), indicative of oxidative and nitrosative chemistry. Infused N-oxides were inactive as vasodilators whereas S-nitrosoglutathione dilated conduit and resistance arteries. Our results suggest that NO can be transported in bioactive form for significant distances along the vascular bed. Both free NO and plasma nitroso species contribute to the dilation of the downstream vasculature.